1. Field of the Invention
Hardening of aqueous alkali metal silicate solutions with carbonate compositions and their use for soil stabilization.
2. Statement of the Related Art
Aqueous alkali metal silicate solutions for stabilizing soil and waterproofing the ground have long been known and are widely used in practice. Particularly in the construction of subways and other underground railways, alkali metal silicate solutions are injected into the foundation to stabilize the ground and thus to underpin already erected structures.
Thus, U.S. Pat. No. 4,293,340 and corresponding European Patent Application No. 28,495 describe a ground stabilizing composition which, in addition to aqueous alkali metal silicate solutions, contains glyoxal and hydrogen peroxide and, optionally, inorganic salts. Unfortunately, satisfactory compressive strength is not achieved by stabilizing soil with these known compositions.
The hardening of alkali metal silicate aqueous solutions containing inorganic and/or organic mixtures, with esters of organic acids partially is described in German Patent Application Nos. 21 64 957 and 21 65 912 (as well as partially corresponding U.S. Pat. Nos. 3,850,650 and 3,864,137). The hardeners used are alkyl esters of optionally halogen-, cyano- or hydroxy-substituted carboxylic acids, such as formic acid, acetic acid or propionic acid, or of dicarboxylic acids, such as oxalic acid, succinic acid or adipic acid.
However, the use of the esters mentioned above results in serious pollution of the subsoil and ground water with organic substances which are released in the course of the setting reaction. This pollution is reflected in an increase in the chemical oxygen demand (COD) value which indicates the degree of pollution of water by organic compounds. The use of compounds which greatly increase that value is unacceptable.
German Patent Application No. 21 64 957 also describes the use of dimethyl carbonate as a hardener for alkali metal silicate solutions which are used in a process for the production of foam waterglass moldings. In processes such as these, short pot lives, i.e. short periods of time in which the reaction mixture may be processed in accordance with the invention, are desirable if not actually necessary. Pot lives of 30 seconds to a maximum of 10 minutes are mentioned. A hardener resulting in a longer than 10 minute pot life is disclosed in this German application as only useful if in admixture with a short pot life hardener, so that the combination results in a pot life of 10 minutes or less. Dimethyl carbonate is disclosed as yielding a pot life of less than 10 minutes, specifically 4.5 minutes.
However, aqueous alkali metal silicate solutions which are to be used for stabilizing and waterproofing soil require pot lives of considerably longer than 10 minutes because otherwise the silicate is in danger of gelling or hardening before the solutions have been introduced into the ground through the injection lances generally used.